Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary
We review previous work of Alain Connes, and its extension by the author, on some conformal invariants obtained from the noncommutative residue on even dimensional compact manifolds without boundary. Inspired by recent work of Yong Wang, we also address possible generalizations of these conformal in...
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irk-123456789-1471972019-02-14T01:27:06Z Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary Ugalde, W.J. We review previous work of Alain Connes, and its extension by the author, on some conformal invariants obtained from the noncommutative residue on even dimensional compact manifolds without boundary. Inspired by recent work of Yong Wang, we also address possible generalizations of these conformal invariants to the setting of compact manifolds with boundary. 2007 Article Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary / W.J. Ugalde // Symmetry, Integrability and Geometry: Methods and Applications. — 2007. — Т. 3. — Бібліогр.: 23 назв. — англ. 1815-0659 2000 Mathematics Subject Classification: 53A30 http://dspace.nbuv.gov.ua/handle/123456789/147197 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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We review previous work of Alain Connes, and its extension by the author, on some conformal invariants obtained from the noncommutative residue on even dimensional compact manifolds without boundary. Inspired by recent work of Yong Wang, we also address possible generalizations of these conformal invariants to the setting of compact manifolds with boundary. |
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Ugalde, W.J. Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary Symmetry, Integrability and Geometry: Methods and Applications |
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Ugalde, W.J. |
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Ugalde, W.J. |
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Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary |
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Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary |
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Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary |
| title_fullStr |
Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary |
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Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary |
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some conformal invariants from the noncommutative residue for manifolds with boundary |
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Інститут математики НАН України |
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2007 |
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Some Conformal Invariants from the Noncommutative Residue for Manifolds with Boundary / W.J. Ugalde // Symmetry, Integrability and Geometry: Methods and Applications. — 2007. — Т. 3. — Бібліогр.: 23 назв. — англ. |
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Symmetry, Integrability and Geometry: Methods and Applications |
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AT ugaldewj someconformalinvariantsfromthenoncommutativeresidueformanifoldswithboundary |
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2025-07-11T01:35:48Z |
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2025-07-11T01:35:48Z |
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1837312517607522304 |
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Symmetry, Integrability and Geometry: Methods and Applications SIGMA 3 (2007), 104, 18 pages
Some Conformal Invariants from the Noncommutative
Residue for Manifolds with Boundary?
William J. UGALDE
Escuela de Matemática, Universidad de Costa Rica, Código postal 2060 San José, Costa Rica
E-mail: william.ugalde@ucr.ac.cr
URL: http://www2.emate.ucr.ac.cr/∼ugalde/
Received August 06, 2007, in final form October 31, 2007; Published online November 07, 2007
Original article is available at http://www.emis.de/journals/SIGMA/2007/104/
Abstract. We review previous work of Alain Connes, and its extension by the author, on
some conformal invariants obtained from the noncommutative residue on even dimensional
compact manifolds without boundary. Inspired by recent work of Yong Wang, we also
address possible generalizations of these conformal invariants to the setting of compact
manifolds with boundary.
Key words: manifolds with boundary; noncommutative residue; Fredholm module; confor-
mal invariants
2000 Mathematics Subject Classification: 53A30
1 Introduction
There is one particular aspect of noncommutative geometry that has historically received less
attention than other of its subjects; the use of its machinery to obtain conformal invariants
(associated to the underlying manifold). The motivating example in this venue, is a conformal
invariant in dimension 4 (Connes [5]) and its extension to higher order even dimensional mani-
folds by the author [19]. The main idea lies on Theorem IV.4.2.c of Connes [6]. This theorem
states that the oriented conformal structure of a compact even-dimensional smooth manifold is
uniquely determined by the Fredholm module (H, F, γ) of Connes, Sullivan and Teleman [7]; via
the noncommutative residue Res of Adler, Manin, Guillemin, and Wodzicki [1, 13, 12, 23].
In Section 2 of [5] Connes uses his quantized calculus to find a conformal invariant in the
4-dimensional case. A central part of the explicit computation of this conformal invariant is the
study of a trilinear functional on smooth functions over the manifold M4 given by the relation
τ(f0, f1, f2) = Res(f0[F, f1][F, f2]).
Here F is a pseudodifferential operator of order 0 acting on 2-forms over M4. This conformal
invariant computed by Connes in the 4-dimensional case is a natural bilinear differential func-
tional of order 4 acting on C∞(M4). In [5] it is denoted Ω and in these notes it is denoted
B4 dx.
This bilinear functional is symmetric, B4(f1, f2) = B4(f2, f1), and conformally invariant, in
the sense that B̂4(f1, f2) = e−4ηB4(f1, f2) for a conformal change of the metric ĝ = e2ηg. It is
also uniquely determined by the relation:
τ(f0, f1, f2) =
∫
M
f0B4(f1, f2) dx, ∀ fi ∈ C∞(M4).
?This paper is a contribution to the Proceedings of the 2007 Midwest Geometry Conference in honor of
Thomas P. Branson. The full collection is available at http://www.emis.de/journals/SIGMA/MGC2007.html
mailto:william.ugalde@ucr.ac.cr
http://www2.emate.ucr.ac.cr/~ugalde/
http://www.emis.de/journals/SIGMA/2007/104/
http://www.emis.de/journals/SIGMA/MGC2007.html
2 W.J. Ugalde
Furthermore, in the 4-dimensional case, Connes has also shown that the Paneitz operator [14]
(critical GJMS for n = 4 [10]), can be derived from B4 by the relation∫
M
B4(f1, f2) dx =
1
2
∫
M
f1P4(f2) dx.
Aiming to extend the work of Connes to even dimensional manifolds, in [18] we have proved
the following two results:
Theorem 1 of [18]. Let M be an n-dimensional compact conformal manifold without boundary.
Let S be a pseudodifferential operator of order 0 acting on sections of a vector bundle over M such
that S2f1 = f1S
2 and the pseudodifferential operator P = [S, f1][S, f2] is conformally invariant
for any fi ∈ C∞(M). Then there exists a unique, symmetric, bilinear, differential functional
Bn,S of order n conformally invariant in the sense that B̂n,S(f1, f2) = e−nηBn,S(f1, f2), for
ĝ = e2ηg, and such that
Res(f0[S, f1][S, f2]) =
∫
M
f0Bn,S(f1, f2) dx
for all fi ∈ C∞(M).
A particular case of the above result occurs when one works with even-dimensional manifolds
for then, it makes sense to consider the Fredholm module (H, F ) associated to M. The operator
F has the property F 2 = 1 and in general [F, f ] 6= 0 for f ∈ C∞(M). Taking S as F in the
previous theorem one has:
Theorem 2 of [18]. Let M be a compact conformal manifold without boundary of even di-
mension n and let (H, F ) be the Fredholm module associated to M by A. Connes [5]. Then,
by taking S = F in Theorem 1 of [18] there is a unique, symmetric, and conformally invariant
n-differential form Bn = Bn,F such that
Res(f0[F, f1][F, f2]) =
∫
M
f0Bn(f1, f2) dx
for all fi ∈ C∞(M).
These results are based on the study of the formula for the total symbol σ(P1P2) of the
product of two pseudodifferential operators, in the particular case in which one of them is
a multiplication operator. The following is the main result of [19]:
Theorem 1. Let M be a compact conformal manifold without boundary of even dimension n
and let (H, F ) be the Fredholm module associated to M by Connes [5]. Let Pn be the differential
operator given by the relation∫
M
Bn(f, h) dx =
∫
M
fPn(h) dx
for all f, h ∈ C∞(M). Then,
i) Pn is formally selfadjoint;
ii) Pn is conformally invariant in the sense P̂n(h) = e−nηPn(h), if ĝ = e2ηg;
iii) Pn is expressible universally as polynomial in the components of ∇ (the covariant deriva-
tive) and R (the curvature tensor) with coefficients rational in n.
iv) Pn(h) = cn∆n/2(h)+ “lower order terms”, with cn a universal constant;
Conformal Invariants for Manifolds with Boundary 3
v) Pn has the form δSnd where Sn is an operator on 1-forms given as a constant multiple of
∆n/2−1+ “lower order terms” or (dδ)n/2−1+ “lower order terms”;
vi) Pn and Bn are related by:
Pn(fh)− fPn(h)− hPn(f) = −2Bn(f, h).
Because the critical GJMS operator and the operator Pn coincide in the flat case and share
the same conformal behavior we have
Proposition 1. In the even dimensional case, inside the conformally flat class of metrics, the
critical GJMS operator and the operator Pn coincide up to a constant multiple.
1.1 Yong Wang’s work
Based on the work of [20], in [21] and [22] Y. Wang proposes to extend to the case of manifolds
with boundary, the work of Connes in Section 2 of [5]. He is the first to suggest the replacement of
the usual noncommutative residue by the noncommutative residue of Fedosov–Golse–Leichtnam–
Schrohe [8], acting on Boutet de Monvel’s algebra [3].
In Section 3 of [21], Y. Wang considers a compact n-dimensional manifold X with boundary Y
and its double manifold X̃ = X ∪Y X. For a vector bundle E over X̃ and a pseudodifferential
operator S with the transmission property and of order 0 acting on sections of E, the operator P̃
is defined as the composition
P̃ :=
(
π+f0 0
0 0
) [(
π+S 0
0 0
)
,
(
π+f1 0
0 0
)] [(
π+S 0
0 0
)
,
(
π+f2 0
0 0
)]
.
It is then observed that P̃ = π+(f0[S, f1][S, f2]) + G for some singular Green operator G with
singular Green symbol b. See [21] for the corresponding definitions. Based on this decomposition
of P̃ and the definition of Fedosov et al. of Res, Wang defines Ωn,S and Ωn−1,S via
Ωn,S(f1, f2) = res
(
([S, f1][S, f2])|X
)
and f0|Y Ωn−1,S(f1, f2) = 2π resx′ trace(b),
with f i a smooth extension of fi to X̃. Also res is the density corresponding to Res (similar to
res for Res) and resx′ the noncommutative residue density for the manifold Y.
It is possible to verify that P̃ satisfies the transmission property. In this way, forgetting
about any conformal invariance property, in [21] Wang found a generalization of the relation
Res(f0[S, f1][S, f2]) =
∫
M
f0Ωn,S(f1, f2) =
∫
M
f0Bn,S(f1, f2) dx, ∀ fi ∈ C∞(X ∪ Y )
in Theorem 1 of [18].
The main idea of Y. Wang relies on the use of the double manifold. Topologically, the double
manifold of a given compact oriented manifold with boundary makes perfect sense. At the level
of smooth manifold with a given Riemannian structure more work is needed to make sense of
a double manifold.
In Section 4 of [21] the dimension is taken as even and the metric on X has a product structure
near the boundary: gX = g∂X + dxn. On X̃ the metric g̃ is taken as g̃ = g on both copies of X.
Then Ωn is defined as Ωn,F (f1, f2)|X where (H, F ) is the Fredholm module associated to (X̃, g̃),
and fi is an extension of fi to M̃.
In [22] the even-dimensional Riemannian metric in consideration has the particular form
gX = 1/(h(xn))g∂X + dx2
n on a collar neighborhood U of ∂X. Here h is the restriction to [0, 1)
of a smooth function h̃ on (−ε, 1) for some ε > 0 such that h(0) = 1 and h(xn) > 0. A metric ĝ
4 W.J. Ugalde
is associated to the double manifold X̃ in the following way. Given U and h̃ as before, there is
a metric ĝ on X̃ with the form ĝX̃ = 1/(h̃(xn))g∂X + dx2
n on U ∪∂X ∂X × (−ε, 0] and such that
ĝ|X = g.
Next, with (H, Fĝ) the Fredholm module associated to (X̃, ĝ), Ωn and Ωn−1 are defined via
the relation
Res
(
π+f0[π+Fĝ, f1][π+Fĝ, f2]
)
=
∫
X
f0Ωn(f1, f2)(ĝ) +
∫
∂M
f0|∂XΩn−1(ĝ).
The described settings used in [21] and [22] have the following limitations: if we conformally
rescale the metric in X then, the new metric e2ηg is not anymore of the specific requested form
near the boundary. How to define then the objects in question in terms of this new metric? That
is to say, what is the definition of Ωn(e2ηg) and how to compare it with Ωn(g)? Based on the
idea of replacing Res with Res, and inspired by the work of Wang, we propose the approach in
this work to the problem of extending the results in Section 2 of [5] to manifolds with boundary.
1.2 Contents
We first review the construction of the even Fredholm module (H, F, γ) over the commutative
algebra A = C∞(M) (trivially an involutive algebra over C) of smooth functions over a compact
oriented manifold M without boundary. We give special attention to its conformal properties.
Then we review the statement of Connes about recovering the conformal structure from this
Fredholm module and a recent characterization on the subject by Bär. Next, we move to the
setting of manifolds with boundary. Aiming to extend previous work, we briefly review the
noncommutative residue for manifolds with boundary and Boutet de Monvel calculus according
to our needs. Last, we present a couple of results that extend to the even dimensional case
Theorems 1 and 2 in [18] to the following setting of manifolds with boundary: M is a compact
manifold with boundary ∂M such that M is embedded in a compact oriented manifold M̃
without boundary. Further we assume Riemannian structures (M̃, g̃) and (M, g) such that g
coincides with g̃ restricted to M. The results are Theorems 4 and 5 respectively.
1.3 Other possibilities
For a Riemann surface M , a map f = (f i) from M to R2 and metric gij(x) on M , the 2-di-
mensional Polyakov action [15] is given by
I(f) =
1
2π
∫
M
gij df i ∧ ?df j .
By considering instead of df its quantized version [F, f ], Connes [5] quantized the Polyakov
action as a Dixmier trace:
1
2π
∫
M
gijdf
i ∧ ?df j = −1
2
Trω
(
gij [F, f i][F, f j ]
)
.
Connes’ trace theorem [4] states that the Dixmier trace and the noncommutative residue of an
elliptic pseudodifferential operator of order−n on an n-dimensional manifold M are proportional
by a factor of n(2π)n. In the 2-dimensional case the factor is 8π2 and so, the quantized Polyakov
action can be written as
−16π2I = Res
(
gij [F, f i][F, f j ]
)
.
This quantized Polyakov action makes sense in the general even dimensional case.
Conformal Invariants for Manifolds with Boundary 5
Although this generalization of the Polyakov action motivates the particular form of the
functional Res(f0[F, f1][F, f2]), the same Fredholm module yields other functionals in dimensions
greater than 4. For instance, for dimension 6 one could also consider
Res(f0[F, f1][F, f2][F, f3]) =
∫
M
f0 T (f1, f2, f3) d6x,
which is a Hochschild 3-cocycle and the trilinear expression T (f1, f2, f3) is conformally invariant.
In greater 2l dimensions,
Res(f0[F, f1] · · · [F, fl]) =
∫
M
f0 C(f1, . . . , fl) d2lx,
invites to study the role of the conformal invariant C(f1, . . . , fl).
1.4 Further directions
The possibility of obtaining from the expression∫
M
Bn(f1, f2
)
dx + 2π
∫
∂M
∂Bn(f1|∂M , f2|∂M ) dx′
=
∫
M
f1Pnf2 dx +
∫
∂M
f1|∂MP ′
n−1f2|∂M dx′
conformally covariant differential operators Pn and P ′
n−1 acting on M and ∂M respectively (in
a way similar to the boundaryless case), is the motivating force behind this project. We hope
to report on that in the near future.
One more possibility is to study a Riemannian manifold with a particular metric structure
near the boundary, in such a way that it makes sense to consider its double manifold and at the
same time, study conformal variations of the metric. One can ask what sort of specific objects are
to be found using the ideas presented here in such a particular situation. A seemingly promising
case is that of manifolds with totally geodesic boundaries, for which the double manifold is
natural to be considered.
2 The Fredholm module for a conformal manifold
Following Definition IV.4.1 [6], an even Fredholm module (H, F, γ) is given by
• An involutive algebra A (over C) together with a Hilbert space H and an involutive
representation π of A in H.
• An operator F on H such that F = F ∗, F 2 = 1, and [F, π(a)] is a compact operator
∀ a ∈ A.
• A Z/2 grading γ, γ = γ∗, γ2 = 1 of H such that γπ(a) = π(a)γ, ∀ a ∈ A, and γF = −Fγ.
In the case of a manifold without boundary, the very first ingredient in (H, F, γ) is the
involutive algebra A = C∞(M) where we allow complex values. The fact that, for an oriented
Riemannian manifold M (with or without boundary) and of even dimension n, the restriction of
the Hodge star operator to middle-dimension forms is conformally invariant, is central to what
follows. We consider the vector bundle Ωn/2
C (M) of complex middle-dimension forms. We drop
the subscript C from now on. In this way, for an n-dimensional oriented compact manifold,
n even, the space Ωn/2(M) of (complex) middle dimension forms has a (complexified) inner
product
〈ω1, ω2〉 =
∫
M
ω1 ∧ ?ω2.
6 W.J. Ugalde
This inner product is unchanged under a conformal change of the metric and so, its Hilbert
space completion H0 = L2(M,Ωn/2(M)) depends only on the conformal class of the metric.
H0 is by construction a C∞(M)-module with (fω)(p) = f(p)ω(p) for all f ∈ C∞(M), ω ∈ H0,
and p ∈ M .
If M is a compact manifold without boundary then, the harmonic forms (those in the kernel
of ∆) are precisely those in Ker d∩Ker δ. If M is even dimensional with dimension n, the Hodge
decomposition for middle-dimension forms looks like
Ωn/2(M) = ∆(Ωn/2(M))⊕Hn/2 = d(Ωn/2−1(M))⊕ δ(Ωn/2+1(M))⊕Hn/2.
Here Hn/2 = Kern/2 ∆. Thus H0 is the direct sum of Hn/2 and the images of d and δ.
The Hilbert space H is H = H0⊕Hn/2, the direct sum of H0 with an extra copy of the finite
dimensional Hilbert space of harmonic middle-dimension forms on M .
For each f ∈ A = C∞(M) we consider the multiplication operator on H0, f : ω 7→ fω. These
multiplication operators on H0 do not preserve the subspace of harmonic forms. Thus the extra
copy of Hn/2 in H is to preserve the notion of Z2-graded Hilbert space.
The Hilbert space representation of C∞(M) inH is given by f 7→ π(f) with π(f)(ω+h) := fω,
for all ω ∈ H0 and h ∈ Hn/2. Evidently, π(f) is a bounded operator on H. It is not difficult
to verify that π is an involutive representation of C∞(M) in H. To simplify the notation we
write f instead of π(f).
Next we look at the Z/2 grading. Because the Hodge star operator ? acting on middle-
dimension forms satisfies ?2 = (−1)n/2, the operator
γ0 := (−1)
(n/2)(n/2−1)
2 in/2?
is of square one giving a Z2-grading on H0. Since ?∗ = (−1)n/2? when acting on middle forms
we have
γ∗0 := (−1)
(n/2)(n/2−1)
2 (−i)n/2?∗ = (−1)
(n/2)(n/2+3)
2 in/2? = γ0. (1)
It follows from the definition of γ0 that
Lemma 1. The operator γ0 exchanges the subspaces d
(
Ωn/2−1
)
and δ
(
Ωn/2+1
)
and their clo-
sures. As a consequence and because of (1), γ0(Hn/2) = Hn/2.
We define γ : H0 ⊕Hn/2 → H0 ⊕Hn/2 by γ(ω + h) := γ0(ω) − γ0h. In this way, the extra
copy of Hn/2 is endowed with the opposite Z2-grading −γ: Hn/2± = {h ∈ Hn/2 : γh = ∓h} and
so H has the Z2-grading given by H+ = H+
0 ⊕Hn/2+
and H− = H−
0 ⊕Hn/2−.
It is straightforward to verify that the operator γ defined on H satisfies γ = γ∗, γ2 = 1, and
γf = fγ for all f ∈ C∞(M).
The first step to define the operator F is the following observation
Lemma 2. For ω = dβ + δβ′ ∈ d(Ωn/2−1(M)) ⊕ δ(Ωn/2+1(M)), the operator F0 : H0 → H0
defined by F0(dβ + δβ′) := dβ − δβ′ and extended as zero over Hn/2 is a partial isometry such
that F 2
0 = 1 on H0 Hn/2, and F0 is its own formal adjoint operator. Furthermore, 1− F 2
0 is
the orthogonal projection on the finite-dimensional Hilbert space of middle-dimension harmonic
forms.
The operator F is defined on H = d(Ωn/2−1(M))⊕ δ(Ωn/2+1(M))⊕Hn/2 ⊕Hn/2 by
F =
F0 0 0
0 0 1
0 1 0
. (2)
From the previous lemma F ∗ = F and F 2 = 1.
Conformal Invariants for Manifolds with Boundary 7
For an even dimensional oriented compact manifold without boundary, both the Hilbert
space H and the operator F are conformally invariant, thanks to the fact: for a k-form ρ,
δ̂ρ = e−(n−2(k−1))η δ e(n−2k)ηρ.
It is not difficult to verify that γF = −Fγ. Last, since each [F, f ] is a pseudodifferential operator
of order −1 for all f ∈ A = C∞(M), the operator [F, f ] is a compact operator on H via Rellich’s
theorem [9, p. 306].
2.1 Recovering the conformal structure
Theorem IV.4.2.c of [6] states that the Fredholm module (H, F ) uniquely determines the confor-
mal structure of M . For that, Connes uses his trace theorem and the noncommutative residue
to recover the Ln-norm for exterior 1-forms over the manifold.
The first step is to consider instead of df its quantized version [F, f ]. Since F is a pseu-
dodifferential operator of order 0, [F, f ] is a pseudodifferential operator of order −1 for all
f ∈ C∞(Mn), acting on the same vector bundle Ωn/2M as F. The leading symbol of F is given
by
σ0(F )(x, ξ) = |ξ|−2
(
εn
2−1(ξ)ιn
2
(ξ)− ιn
2 +1(ξ)εn
2
(ξ)
)
for all (x, ξ) ∈ T ∗M , ξ 6= 0. Here εk(ξ) and ιk(ξ) represent the exterior and interior multiplication
by the 1-form ξ on k-forms. Note how σ0(F ) does not depend on x ∈ M. The principal symbol
of [F, f ] is
σ−1([F, f ])(x, ξ) = −i
n∑
k=1
∂xkf ∂ξk
(σ0(F ))
which by the expression for σ0(F )(x, ξ) depends only on the value on x of the 1-form df =∑
∂xkf dxk. The details of these statements can be seen for example in [20]. Next for fi ∈
C∞(M) the operator (f1[F, f2])n is a pseudodifferential operator of order −n.
What Theorem IV.4.2.c [6] shows is that Res(f1[F, f2])n and
∫
M ||f1df2||n dx, the Ln-norm
for 1-forms, are proportional.
In the setting of spin Riemannian manifolds, for the algebra C∞(M) of smooth complex
valued functions, the Hilbert space is chosen to be H = L2(M,ΣM), the square integrable
complex spinor fields, and for F one considers the sign of the Dirac operator D. Recently,
Bär [2] showed the following result.
Theorem 2. Let M be a compact spin Riemannian manifold. Let g and g′ be Riemannian
metrics on M and let (H, sign(D)) and (H′, sign(D′)) be the corresponding Fredholm modules of
the algebra C∞(M). Then g and g′ are conformally equivalent if and only if (H, sign(D)) and
(H′, sign(D′)) are weakly unitarily equivalent. That is to say, there is a unitary isomorphism
U : H → H′ such that D′−UDU−1 is a compact operator and for all f ∈ C∞(M) and all h ∈ H
one has U(fh) = fU(h).
The idea is based on the commutativity of U with the action of C∞(M) which implies
that U is induced by a (a.e. invertible) section Ψ of L∞(M,Hom(ΣM,Σ′M)). The principal
symbol of a Dirac operator is given by Clifford multiplication with respect to the metric g,
σD(ξ) = icg(ξ), for all ξ ∈ T ∗M. Because of the relation cg(ξ)cg(η) + cg(η)cg(ξ) = −2g(ξ, η),
for all ξ, η ∈ T ∗M, the principal symbol of sign(D) is
σsign(D)(ξ) =
icg(ξ)
||ξ||g
, ∀ ξ ∈ T ∗M \ {0}.
8 W.J. Ugalde
Since D′ and UDU−1 differ by a compact operator, they have the same sign and thus
cg′(ξ)
||ξ||g′
= Ψ(x)
cg(ξ)
||ξ||g
Ψ−1(x)
for all nonzero ξ ∈ T ∗M. Last
−2g′(ξ, η)
||ξ||g′ ||η||g′
=
cg′(ξ)cg′(η) + cg′(η)cg′(ξ)
||ξ||g′ ||η||g′
= Ψ(x)
(
cg(ξ)cg(η) + cg(η)cg(ξ)
||ξ||g||η||g
)
Ψ−1(x) =
−2g(ξ, η)
||ξ||g||η||g
since the term in the middle is a scalar.
It is important to recall here the result of Connes (see for example [6, p. 544]) that says
that one recovers the metric distance between points in a connected manifold (M, g) from the
relation
d(x, y) = sup{f(x)− f(y) : f ∈ C∞(M) with ||[D, f ]|| ≤ 1}.
Note how with the stronger requirement D′ = UDU−1 (unitarily equivalent) then ||[D, f ]|| =
||[D′, f ]|| and thus d = d′.
If the conformal geometry of (M, [g]) is encoded in the Fredholm module (H, F, γ) over the
algebra C∞(M), then how can one extract the conformal geometry from this Fredholm module?
One possibility is to use it to find conformal invariants associated to a given conformal
manifold, for example, as in the introduction.
3 The noncommutative residue for manifolds with boundary
Remark 1. Wodzicki: (see e.g. [16]) There is no non-zero trace on the algebra of classical
pseudodifferential operators mod the ideal of smoothing operators Ψ∞(M)/Ψ−∞(M), whenever
M is noncompact or has a boundary.
The noncommutative residue of Fedosov–Golse–Leichtnam–Schrohe [8] for manifolds with
boundary is the unique (up to a constant multiple) continuous trace for the operators in Boutet
de Monvel’s algebra. Roughly speaking, this noncommutative residue acts on operators A that
are described by pairs of symbols {ai, ab} called interior and boundary symbol respectively. In
case the manifold has empty boundary this noncommutative residue coincides with the usual
noncommutative residue of Wodzicki, Guillemin, Adler, and Manin.
The setting for the noncommutative residue is given by a compact manifold M with bound-
ary ∂M such that M is embedded in a compact manifold M̃ without boundary, both M and M̃
of dimension n > 1. For M we consider in a boundary chart local coordinates given by (x′, xn)
with x′ = (x1, . . . , xn−1) coordinates for ∂M and xn the geodesic distance to ∂M. It is impor-
tant to mention that the geodesic coordinate chosen for xn is only a technical tool since the
noncommutative residue is independent of the metric and of local representations.
3.1 Boutet de Monvel’s sub-algebra of diagonal symbols
In [3, 8, 11], and [16] one can find detailed introductions to Boutet de Monvel’s calculus. The
operators in Boutet de Monvel’s algebra we are interested in are diagonal matrices of operators
(endomorphisms) A acting on sections of vector bundles E over M and E′ over ∂M :
A =
(
rMPeM + G 0
0 S
)
:
C∞(M,E)
⊕
C∞(∂M, E′)
→
C∞(M,E)
⊕
C∞(∂M, E′)
.
Conformal Invariants for Manifolds with Boundary 9
They are better described by a pair of symbols (ai, ab) where ai is called the interior symbol
and ab is called the boundary symbol. According to our needs, the characterization of such an
operator (or its symbol) of order m is as follows.
P. The operator P is a classical pseudodifferential operator of order m on M̃. Further-
more, P has the so called transmission property. This guarantees that the composition of
different elements remains inside the algebra. Analytically, in local coordinates near ∂M
the transmission property is given by
∂k
xn∂α
ξ′pj(x′, 0, 0,+1) = (−1)j−|α|∂k
xn∂α
ξ′pj(x′, 0, 0,−1), ∀ j, k, α.
Here pj is the homogeneous component of order j in the symbol expansion of the symbol p
of P. Last, eM is the extension by zero of functions (or sections) on M to functions (or
sections) on M̃ and rM is the restriction from M̃ to M. The interior symbol ai of A is
precisely p.
With F we denote the Fourier transform. Also
H+ = {F(χ]0,∞[u) : u is a rapidly decreasing function on R},
H−
0 = {F((1− χ]0,∞[)u) : u is a rapidly decreasing function on R},
H− = H−
0 ⊕ {all polynomials}.
The (diagonal) boundary symbol ab is given by a pair of symbols b, s of operators G, S
parametrized by T ∗∂M \ {0} and the restriction of p to the boundary.
G. The operator G is given by a singular green operator-symbol b(x′, ξ′, Dn) in the following
way. For every l and fixed x′, ξ′,
bl(x′, ξ′, ξn, ηn) ∈ H+⊗̂πH−.
With ⊗̂π we denote Grothendieck’s completion of the algebraic tensor product. The ope-
rator b(x′, ξ′, Dn) : H+ → H+ is given by
[b(x′, ξ′, Dn)h](ξn) = Π′
ηn
(
b(x′, ξ′, ξn, ηn)h(ηn)
)
= lim
ηn→0+
F−1(b(x′, ξ′, ξn, ·)h(·))(ηn).
The operator G described by this operator-symbol b(x′, ξ′, Dn) between functions on [0,∞[
that are rapidly decreasing at ∞, defines a trace class operator on L2(R+). The trace is
given by
trace(G)(x′, ξ′) =
1
2π
∫
b(x′, ξ′, ξn, ξn) dξn.
Note that this is actually a symbol itself.
S. The operator S is a classical pseudodifferential operator of order m along the boundary. It
has values in L(Ck) and each component sj of its symbol expansion s acts by multiplication
on Ck.
The (diagonal) boundary symbol ab is then
ab(x′, ξ′, ξn, ηn) =
(
p(x′, 0, ξ′, ξn) + b(x′, ξ′, ξn, ηn) 0
0 s(x′, ξ′)
)
10 W.J. Ugalde
with
b(x′, ξ′, ξn, ηn) ∼
m∑
l=−∞
bl(x′, ξ′, ξn, ηn) and s(x′, ξ′) ∼
m∑
l=−∞
sl(x′, ξ′)
where for λ > 0
bl(x′, λξ′, λξn, ληn) = λlbl(x′, ξ′, ξn, ηn), sl(x′, λξ′) = λlsl(x′, ξ′).
By Bm
D (M) we denote the collection of all operators of order m with diagonal boundary symbol
and by B∞D (M) the union of all the Bm
D (M). The intersection over all orders m of Bm
D (M) is
denoted B−∞D (M). Last BD = B∞D (M)/B−∞D (M).
Given two operators A1 and A2 in BD with symbols (ai1, ab1) and (ai2, ab2), with entries in
the boundary symbols bj , sj , for j = 1, 2, the composition is again an operator in BD with
symbol (ai, ab) where ai is the usual composition of symbols ai = ai1 ◦ ai2. It also satisfies the
transmission property.
The resulting boundary symbol is of the form
ab = ab1 ◦′ ab2 +
(
L(pi1, pi2) + p+
i1 ◦′ b2 + b1 ◦′ p+
i2 0
0 0
)
.
The symbol ◦′ denotes the usual composition of pseudodifferential symbols on the variables
(x′, ξ′). The terms in the second summand represent the portion on the boundary symbol coming
from the interior symbols. Here, we have hidden in ab1 ◦′ ab2 the part corresponding to the
restriction to the boundary of the interior symbol.
The so called “left-over term” L(pi1, pi2), reflects the particular way the pseudodifferential
operators PM = rMPeM act on the manifold with boundary M. If P1 and P2 are two pseudo-
differential operators on M̃, the difference (P1P2)M − (P1)M (P2)M is a singular Green operator
with associated singular Green operator-symbol L(p1, p2). Since this left-over term need not be
zero, we can not reduce the diagonal sub-algebra by requesting G = 0 in all the operators.
As an example, and because they will be needed later on, let us look at L(f, q) and L(p, f)
where p and q are the symbols of pseudodifferential operators P and Q on M̃, and f ∈ C∞(M̃),
i.e. f represents the pseudodifferential operator on M̃ given multiplication by f. Among all the
possible formulae for L(p, q) available in the literature we decided to use the one provided in [11].
In Section 3 of [11] one can read an explicit expression for L(p, q) in which the effects of p
and q are neatly separated. This expression uses singular Green operators G+(p) and G−(q)
natural for the calculus in use (see Theorems 3.2 and 3.4 [11]).
We content ourselves by quoting a particular situation. By Theorem 3.4 [11], G−(f) = 0 and
by (3.16) [11], L(p, f) = G+(p)G−(f), thus L(p, f) = 0.
Now, for L(f, q) we must look at Theorem 3.5 [11]. In general,
L(p, q) = G+(p)G−(q) +
∑
0≤m< order of Q
Kmγm
where the Km are operators obtained from symbols of a particular type known as Poisson
symbols. By (3.35) [11], Km = 0 when p = f since it depends on higher derivatives on ξn. Since
by Theorem 3.2 [11], G+(f) = 0 we conclude that L(f, p) = 0 as well.
Lemma 3. For every f ∈ C∞(M̃) and every pseudodifferential operator P on M̃ with symbol p,
both left-over terms L(f, p) and L(p, f) vanish.
Last, the operator p+(x′, ξ′, Dn) : H+ → H+ is induced from the action (of the interior
symbol) in the normal direction for fixed (x′, ξ′). The only case we will be interested in are those
of the form f+ ◦′ b2 where f is a smooth function on M̃. We will address them in (3).
Conformal Invariants for Manifolds with Boundary 11
3.2 The noncommutative residue
On Rn with coordinates ξ1, . . . , ξn we consider the (n− 1)-form
σ =
n∑
j=1
(−1)j+1ξj dξ1 ∧ · · · ∧ d̂ξj ∧ · · · ∧ dξn,
where the hat indicates this factor is omitted. Restricted to the unit sphere Sn−1, σ gives the
volume form on Sn−1 and in general dσ = n dξ1 ∧ · · · ∧ dξn. For a coordinate chart U, the form
dx1 ∧ · · · ∧ dxn defines an orientation on U and induces the orientation dξ1 ∧ · · · ∧ dξn on Rn.
For a closed compact manifold M without boundary, the noncommutative residue is defined
as the unique trace (up to constant multiples) on the algebra Ψ∞/Ψ−∞ of classical pseu-
dodifferential operators mod the ideal of smoothing operators.
The following is the main result of [8]:
Theorem 3 (Fedosov–Golse–Leichtnam–Schrohe). Let M be a manifold of dimension n
with smooth boundary ∂M , and let M ∪ ∂M be embedded in a connected manifold M̃ of dimen-
sion n. Let
A =
(
rMPeM + G K
T S
)
be an element in B∞(M)/B−∞(M), with B∞(M) the algebra of all operators in Boutet de Mon-
vel’s calculus (with integral order), B−∞(M) the ideal of smoothing operators, and let p, b, and s
denote the local symbols of P , G, and S respectively. Then
ResA =
∫
M
∫
Sn−1
TrE p−n(x, ξ)σ(ξ) dx
+ 2π
∫
∂M
∫
Sn−2
{
TrE′(trace b−n)(x′, ξ′) + TrE′ s1−n(x′, ξ′)
}
σ′(ξ′) dx′,
with σ′ the n − 2 analog of σ, is the unique continuous trace (up to constant multiples) on the
algebra B∞(M)/B−∞(M).
This trace reduces to the noncommutative residue (of Adler, Manin, Guillemin, and Wodzicki)
in the case ∂M = ∅, and it is independent of the Riemannian metric (eventually) chosen on M.
4 On manifolds with boundary
In this section we present an extension of Theorem 1 in [18] to the setting of manifolds with
boundary. Let M be a manifold with boundary ∂M. Assume that the compact manifold M is
embedded in a compact manifold M̃ without boundary. Further we assume M̃ to be oriented
which determines an orientation on M and thus on ∂M.
For P a pseudodifferential operator acting on a vector bundle E over M̃ with symbol p
having the transmission property up to the boundary, S a pseudodifferential operator acting on
a vector bundle E′ over ∂M with symbol s, and for f ∈ C∞(M̃) we let A(P, S) and A(f) be the
elements in Boutet de Monvel’s algebra of diagonal elements given by
A(P, S) =
(
rMPeM + 0 0
0 S
)
, A(f) =
(
rMfeM + 0 0
0 f |∂M
)
.
We study Res(A(f0)[A(P, S), A(f1)][A(P, S), A(f2)]) for functions fi ∈ C∞(M̃).
12 W.J. Ugalde
First of all, we must check that this product operator remains inside the calculus in use. It
follows from Proposition 2.7 of [17], which states that if two operators satisfy the transmission
property then their product satisfies the transmission property as well.
Since L(f, p) = 0 = L(p, f), for all f ∈ C∞(M̃) it follows that
A(f0)[A(P, S), A(f1)][A(P, S), A(f2)] =(
rMf0[P, f1][P, f2]eM + f+
0 ◦′ L(σ([P, f1]), σ([P, f2])) 0
0 f0|∂M ◦′ [S, f1|∂M ]′ ◦′ [S, f2|∂M ]′
)
where ◦′ represents the symbol composition with respect to (x′, ξ′). Here σ([P, fi]) represents as
usual the symbol of the operator [P, fi]. Using the definition of Res for manifolds with boundary
we have
Res
(
A(f0)[A(P, S), A(f1)][A(P, S), A(f2)]
)
=
∫
M
∫
Sn−1
TrE
{
σ−n
(
f0[P, f1][P, f2](x, ξ)
)}
σ(ξ) dx
+ 2π
∫
∂M
∫
Sn−2
TrE′
{
σ−(n−1)
(
trace
{
f+
0 ◦′ L(σ([P, f1]), σ([P, f2]))
}
(x′, ξ′)
)}
+ TrE′
{
σ−(n−1)
(
f0|∂M ◦′ [S, f1|∂M ]′ ◦′ [S, f2|∂M ]′(x′, ξ′)
)}
σ′(ξ′) dx′.
4.1 A pair of bilinear functionals
Mimicking the boundaryless case and following [21] we define:
Definition 1.
Bn,P (f1, f2) :=
∫
Sn−1
TrE
{
σ−n
(
[P, f1][P, f2](x, ξ)
)}
σ(ξ),
and
∂Bn,P,S(f1, f2) :=
∫
Sn−2
TrE′
{
σ−(n−1)
(
trace
{
L(σ(([P, f1]), σ(([P, f2]))
}
(x′, ξ′)
)}
+ TrE′
{
σ−(n−1)
((
[S, f1|∂M ]′ ◦′ [S, f2|∂M ]
)
(x′, ξ′)
)}
σ′(ξ′),
for all fi ∈ C∞(M̃).
By definition, both Bn,P and ∂Bn,P,S are bilinear. Since f0 is independent of ξ we have∫
Sn−1
TrE
{
σ−n
(
f0[P, f1][P, f2](x, ξ)
)}
σ(ξ) = f0Bn,P (f1, f2).
The computations done in [19] with the symbol expansions for the case of empty boundary are
also valid here. In particular we have in given local coordinates the explicit expression
Bn,P (f1, f2) =
∑ Dβ
x(f1)Dα′′+δ
x (f2)
α′!α′′!β!δ!
∫
Sn−1
Tr
{
∂α′+α′′+β
ξ (σP
k−i)∂
δ
ξ (D
α′
x (σP
k−j))
}
σ(ξ)
with the sum taken over |α′|+ |α′′|+ |β|+ |δ|+ i+ j = n+2k, |β| ≥ 1, and |δ| ≥ 1. It shows that
Bn,P (f1, f2) is differential in f1 and f2. Evidently it is possible to obtain a similar expression
for the summand in ∂Bn,P,S corresponding to S replacing n by n− 1 and x by x′.
In p. 25 of [8] we can read an expression for the degree −(n− 1) component of the operator-
symbol trace(c) with c = p+ ◦′ b. It is given by
σ−(n−1)
(
trace c(x′, ξ′)
)
∼
∞∑
j=0
ij
j!
Π′
ξn
{
σ−n
(
∂j
ξn
[∂j
xn
p(x′, 0, ξ′, ξn) ◦′ b(x′, ξ′, ξn, ηn)]
)
|ηn=ξn
}
.(3)
Conformal Invariants for Manifolds with Boundary 13
Thus, since f0 is independent of ξ,
trace
{
σ−(n−1)
(
f+
0 ◦′ L(σ([P, f1]), σ([P, f2]))
)}
= f(x′, 0) trace{σ−(n−1)(L(σ([P, f1]), σ([P, f2]))(x′, ξ′)}.
and it follows that∫
Sn−2
Tr
{
σ−(n−1)
(
trace
{
f+
0 ◦′ L(σ([P, f1]), σ([P, f2]))
}
(x′, ξ′)
)}
σ′(ξ′)
= f(x′, 0)
∫
Sn−2
Tr
{
σ−(n−1)
(
trace
{
L(σ([P, f1]), σ([P, f2]))
}
(x′, ξ′)
)}
σ′(ξ′),
for all fi ∈ C∞(M̃). In this way
Res
(
A(f0)[A(P, S), A(f1)][A(P, S), A(f2)]
)
=
∫
M
f0Bn,P (f1, f2
)
dx + 2π
∫
∂M
f0|∂M∂Bn,P,S(f1|∂M , f2|∂M ) dx′. (4)
Lemma 4. The functionals Bn,P and ∂Bn,P,S are bilinear and symmetric.
Proof. The symmetry of both Bn,P and ∂Bn,P,S is not evident from the expressions above.
For Bn,P it was obtained in [20] in the boundaryless case from the trace property of Res .
Because it shares the same local expression both for empty and non-empty boundary we have
that Bn,P is symmetric.
For ∂Bn,P,S we are going to exploit the linearity and the trace property of the noncommutative
residue. Denote f = A(f) and P = A(P, S). Using that f1 f2 = f2 f1 for all fi ∈ C∞(M̃) and the
trace property of the noncommutative residue we have that all of Res
(
f0 f2 P P f1−f0 f1 P P f2
)
,
Res
(
f0 f2 P P f1 − f0 f1 P P f2
)
, and Res
(
f0 P f2 P f1 − f1 f0 P f2 P
)
vanish. In this way
Res(f0[P , f1][P , f2]− f0[P , f2][P , f1])
= Res(f0 P f1 P f2 − f0 P f1 f2 P − f0 f1 P P f2 + f0 f1 P f2 P
− f0 P f2 P f1 + f0 P f2 f1 P + f0 f2 P P f1 − f0 f2 P f1 P )
= Res(f0 P f1 P f2 + f0 f1 P f2 P − f0 P f2 P f1 − f0 f2 P f1 P ) = 0.
Hence∫
M
f0Bn,P (f1, f2) + 2π
∫
∂M
f0|∂M∂Bn,P,S(f1, f2)
=
∫
M
f0Bn,P (f2, f1) + 2π
∫
∂M
f0|∂M∂Bn,P,S(f2, f1), ∀ fi ∈ C∞(M̃).
Since Bn,P (f1, f2) is symmetric∫
∂M
f0|∂MBS(f1, f2) =
∫
∂M
f0|∂MBS(f2, f1), ∀f0 ∈ C∞(M̃)
and the result follows from the arbitrariness of f0. �
Lemma 5. ∂Bn,P,S(f1, f2) is differential on f1 and f2.
14 W.J. Ugalde
Proof. We denote, to simplify the notation, P1 = [P, f1] and P2 = [P, f2] with symbols p1
and p2 respectively. In p. 27 of [8] we can read the following
trace
{
L(p1, p2)
}
(x′, ξ′)
=
∞∑
j,k=0
(−i)j+k+1
(j + k + 1)!
Π′
ξn
(
∂j
xn
∂k
ξn
Π+
ξn
(p1)(x′, 0, ξ′, ξn) ◦′ ∂j+1
xn
∂k
ξn
Π+
ξn
(p2)(x′, 0, ξ′, ξn)
)
,
with Π+
ξn
(s)(·) the projection of the symbol s on H+. The subscript in Π+ indicates the variable
it is acting on. From [20] we know
σ−k([P, f ]) =
k∑
|β|=1
1
β!
Dβ
x(f)∂β
ξ (σP
−(k−|b|))
thus
Π+
ξn
(σ−k([P, f ]))(x′, 0, ξ′, ξn) =
k∑
|β|=1
1
β!
Π+
ξn
(
Dβ
x(f)∂β
ξ (σP
−(k−|b|))
)
(x′, 0, ξ′, ξn)
=
k∑
|β|=1
1
β!
Dβ
x(f)(x′, 0)Π+
ξn
(
∂β
ξ (σP
−(k−|b|))
)
(x′, 0, ξ′, ξn).
Since any ∂j
xnf factors out of Π′
ξn
we conclude the result. �
4.2 Conformal invariance of Bn,P and ∂Bn,P,S
If we further assume Riemannian structures (M, g) and (M̃, g̃) such that g coincides with g̃
restricted to M then, a conformal rescaling of g corresponds to a conformal rescaling of g̃ (by an
appropriate extension of the conformal factor) and a conformal rescaling of g̃ can be restricted
to a conformal rescaling of g. We obtain
Lemma 6. Assume that P and S are such that [P, f1][P, f2] and [S, f1|∂M ][S, f2|∂M ] are con-
formally invariant for all fi ∈ C∞(M̃). Then
B̂n,P (f1, f2)(x) = e−2nη(x)Bn,P (f1, f2)(x)
and
̂∂Bn,P,S(f1, f2)(x′) = e−2(n−1)η(x′,0)∂Bn,P,S(f1, f2)(x′).
Proof. We want to exploit the independence of Res of local representations. We have
Res(A(f0)[A(P, S), A(f1)][A(P, S), A(f2)])
=
∫
M
f0Bn,P (f1, f2) dx + 2π
∫
∂M
f0|∂M∂Bn,P,S(f1|∂M , f2|∂M ) dx′
=
∫
M
f0B̂n,P (f1, f2) d̂x + 2π
∫
∂M
f0|∂M
̂∂Bn,P,S(f1|∂M , f2|∂M ) d̂x′
=
∫
M
f0e
−2nηB̂n,P (f1, f2) dx + 2π
∫
∂M
f0|∂Me−2(n−1)η|∂M ̂∂Bn,P,S(f1|∂M , f2|∂M ) dx′,
where we use ̂ to represent quantities computed with respect to the conformal metric ĝ = e2ηg.
In particular∫
M
f0(x)Bn,P (f1, f2)(x) dx =
∫
M
f0(x)e−2nη(x)B̂n,P (f1, f2)(x) dx
Conformal Invariants for Manifolds with Boundary 15
for all fi ∈ C∞(M̃) with f0|∂M = 0. Thus Bn,P (f1, f2)(x) = e−2nη(x)B̂n,P (f1, f2)(x) for all
x ∈ M. It follows that∫
∂M
f0(x′, 0)B̂n,P (f1, f2)(x′) d̂x′ =
∫
∂M
f0(x′, 0)e−2nη(x′,0)B̂n,P (f1, f2)(x′) dx′
for all fi ∈ C∞(M̃). The result follows from the arbitrariness of f0. �
Remark 2. Note how the same reasoning in the proof above can be used to show the uniqueness
of Bn,P and ∂Bn,P,S satisfying (4).
Summarizing this section we have
Theorem 4. Let M be a compact manifold of dimension n and with boundary ∂M. Assume
that M is embedded in a compact oriented manifold M̃ without boundary. Further assume
Riemannian structures (M̃, g̃) and (M, g) such that g coincides with g̃ restricted to M. Let P
be a pseudodifferential operator acting on a vector bundle E over M̃ having the transmission
property up to ∂M, let S be a pseudodifferential operator acting on a vector bundle E′ over ∂M,
such that [P, f1][P, f2] and [S, f1|∂M ][S, f2|∂M ] are conformally invariant for all fi ∈ C∞(M̃).
Then Bn,P and ∂Bn,P,S given in Definition 1 are conformally invariant in the sense of Lemma 6.
Furthermore, both Bn,P and ∂Bn,P,S are symmetric, bilinear differential functionals uniquely
determined by the relation (4).
5 On even-dimensional manifolds with boundary
Up to this point, we have a generalization of Theorem 1 in [20] to manifolds with boundary in
the setting described above. Next, we want to state a generalization of Theorem 2 in [20] to
this context. In order to do it, we consider the Fredholm module (H, F ) now associated to the
even-dimensional manifold without boundary M̃.
5.1 The symbol of F and the transmission property
If ω = dβ + δβ′ ∈ d(Ωn/2−1(M̃))⊕ δ(Ωn/2+1(M̃)) then
∆F0(dβ + δβ′) = ∆(dβ − δβ′) = dδdβ − δdδβ′ = F0(d(δdβ) + δ(dδβ′)) = F0∆(dβ + δβ′).
It follows
Lemma 7. For an oriented compact manifold without boundary M̃ and of even dimension n,
the relation F0∆ = ∆F0 = dδ − δd holds on d(Ωn/2−1(M̃))⊕ δ(Ωn/2+1(M̃)).
To be able to use a given pseudodifferential operator in the machinery of the noncommutative
residue for manifolds with boundary, it is essential for the operator to enjoy the transmission
property up to the boundary of M.
Because we are interested in F acting on the orthogonal complement of the harmonic forms
on M̃, we abuse of the notation and use freely F for F0. From the relation ∆F = dδ − δd and
the formula for the total symbol of the product of pseudodifferential operators we can compute
the symbol expansion of F. First we note that F is a pseudodifferential operator of order 0.
We know σ(∆F ) = σ(dδ − δd), thus the formula for the total symbol of the product of two
pseudodifferential operators implies
σdδ−δd
2 + σdδ−δd
1 + σdδ−δd
0 = σ(dδ − δd) = σ(∆F ) ∼
∑ 1
α!
∂α
ξ σ(∆)Dα
x (σ(F ))
∼
∑ 1
α!
∂α
ξ (σ∆
2 + σ∆
1 + σ∆
0 )Dα
x (σF
0 + σF
−1 + σF
−2 + · · · ).
Expanding the right hand side into sum of terms with the same homogeneity we conclude:
16 W.J. Ugalde
Lemma 8. In any given system of local charts, we can express the total symbol of F , σ(F ) ∼
σF
0 + σF
−1 + · · · in a recursive way by the formulae:
σF
0 = (σ∆
2 )−1σdδ−δd
2 , σF
−1 = (σ∆
2 )−1
(
σdδ−δd
1 − σ∆
1 σF
0 −
∑
|α|=1
∂α
ξ (σ∆
2 )Dα
x (σF
0 )
)
,
σF
−2 = (σ∆
2 )−1
(
σdδ−δd
0 − σ∆
1 σF
−1 − σ∆
0 σF
0
−
∑
|α|=1
(
∂α
ξ (σ∆
2 )Dα
x (σF
−1) + ∂α
ξ (σ∆
1 )Dα
x (σF
0 )
)
−
∑
|α|=2
1
α!
∂α
ξ (σ∆
2 )Dα
x (σF
0 )
)
,
σF
−r = −(σ∆
2 )−1
(
σ∆
1 σF
−r+1 + σ∆
0 σF
−r+2 +
∑
|α|=1
∂α
ξ (σ∆
2 )Dα
x (σF
−r+1)
+
∑
|α|=1
∂α
ξ (σ∆
1 )Dα
x (σF
−r+2) +
∑
|α|=2
1
α!
∂α
ξ (σ∆
2 )Dα
x (σF
−r+2)
)
,
for every r ≥ 3.
Lemma 2.4 of [17] states that all symbols which are polynomial in ξ have the transmission
property. Thus both ∆ and dδ− δd have the transmission property. Proposition 2.7 in the same
reference states that if two operators satisfy the transmission property then their products, all
their derivatives, and their parametrizes satisfy the transmission property as well. Furthermore,
the same result also states that it is enough to check that each homogeneous component of
the symbol expansion has the transmission property to conclude that the full symbol has the
transmission property.
By Lemma 8, each homogeneous component σF
−k in the symbol expansion of F is given in
terms of derivatives of the homogeneous components of ∆, dδ−δd, σF
0 , . . . , σF
−k+1, and σ2(∆)−1.
By Lemma 2.4 and Proposition 2.7 of [17] it follows that
Lemma 9. The operator F satisfies the transmission property.
5.2 Res(A(f0)[A(F, 0), A(f1)][A(F, 0), A(f2)])
For F given in (2) now for the manifold M̃, and for f ∈ C∞(M̃) we let A(F, 0) and A(f) be the
elements in Boutet de Monvel’s algebra of diagonal elements given by
F = A(F, 0) =
(
rMFeM + 0 0
0 0
)
, f = A(f) =
(
rMfeM + 0 0
0 f |∂M
)
.
Since L(f, σ(F )) = 0 = L(σ(F ), f), it follows that
f0[F , f1][F , f2] =
(
rMf0[F, f1][F, f2]eM + f+
0 ◦′ L(σ([F, f1]), σ([F, f2])) 0
0 0
)
,
where ◦′ represents the symbol composition with respect to (x′, ξ′). As before we define:
Definition 2.
Bn(f1, f2) :=
∫
Sn−1
Tr
{
σ−n
(
[F, f1][F, f2](x, ξ)
)}
σ(ξ),
and
∂Bn(f1, f2) =
∫
Sn−2
Trσ−(n−1)
{(
trace
{
L(σ([F, f1]), σ([F, f2]))(x′, ξ′)
})}
σ′(ξ′),
for all fi ∈ C∞(M̃).
Conformal Invariants for Manifolds with Boundary 17
As in Section 4
Theorem 5. Both differential functionals Bn and ∂Bn are bilinear, symmetric, conformal in-
variant in the sense
B̂n(f1, f2)(x) = e−2nη(x)Bn(f1, f2)(x)
and
̂∂Bn(f1, f2)(x′) = e−2(n−1)η(x′,0)∂Bn(f1, f2)(x′)
for a conformal change of the metric ĝ = e2ηg, and are uniquely determined by the relation
Res
(
A(f0)[A(F, 0), A(f1)][A(F, 0), A(f2)]
)
=
∫
M
f0Bn,P (f1, f2) dx + 2π
∫
∂M
f0|∂M∂Bn,P,S(f1, f2) dx′.
Remark 3. Even though both bilinear functionals Bn and ∂Bn are acting on C∞(M̃), they
depend on the particular embedding of the compact manifold M into M̃, and thus, they can be
defined on C∞(M) by considering an extension of f ∈ C∞(M) to C∞(M̃).
Remark 4. In case M is odd dimensional, all results from the first part of these notes are
valid on the compact even dimensional manifold without boundary ∂M. In this way, we can
consider the commutative algebra A = C∞(∂M) and the Fredholm module associated to the
manifold ∂M. For F given in (2) on the manifold ∂M, we could look at
A(P, F ) =
(
P 0
0 F
)
and try to study Res(A(f0)[A(P, F ), A(f1)][A(P, F ), A(f2)]) for functions fi ∈ C∞(M̃). The
trivial choice P = 0 will produce Bn,P = 0 and ∂Bn,0,F = Bn−1. It is an open problem to search
for a companion P for F that will produce more interesting results in the odd dimensional case.
Acknowledgements
This research is supported by Vicerrectoŕıa de Investigación de la Universidad de Costa Rica
and Centro de Investigaciones Matemáticas y Meta-matemáticas. The material extends a talk
presented in May 2007 at the Midwest Geometry Conference held at the University of Iowa in
honor of Thomas P. Branson.
The referees’ suggestions improved to a great extent the presentation of this material. One of
the referees pointed the author towards [11] which provided a clearer understanding of Boutet
de Monvel’s calculus. In particular, the formulae used for L(p, q) resulted in a significant sim-
plification of the treatment of the subject.
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http://arxiv.org/abs/math.AP/9911053
http://arxiv.org/abs/math.DG/0211240
http://arxiv.org/abs/math.DG/0403392
http://arxiv.org/abs/math.DG/0211361
http://arxiv.org/abs/math.DG/0609062
http://arxiv.org/abs/math.DG/0609060
1 Introduction
1.1 Yong Wang's work
1.2 Contents
1.3 Other possibilities
1.4 Further directions
2 The Fredholm module for a conformal manifold
2.1 Recovering the conformal structure
3 The noncommutative residue for manifolds with boundary
3.1 Boutet de Monvel's sub-algebra of diagonal symbols
3.2 The noncommutative residue
4 On manifolds with boundary
4.1 A pair of bilinear functionals
4.2 Conformal invariance of Bn,P and Bn,P,S
5 On even-dimensional manifolds with boundary
5.1 The symbol of F and the transmission property
5.2 Res ...
References
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